1. Field of Invention
The present invention relates to a transport protocol. More particularly, the invention relates to a transport protocol for communicating control signals from remote control devices, including an infrared keyboard and pointing device, to an electronic device.
2. Related Art
Set-top boxes are utilized by consumers for receiving multimedia signals from service providers such as satellite and cable. The set-top box is also capable of receiving signals from remote control devices, such as a television remote control, as well as a keyboard or pointing device. These remote devices allow a subscriber of services (user) to interface with the multimedia or computer software that is viewed on a display panel that is coupled to the set-top box. The keyboard and pointing device (e.g., mouse) interface with a processor in the set-top box via pulsed infrared (IR) signals. However, signal protocols used by such remote control devices, in particular infrared protocols, may not have enough flexibility to support the number of keycodes needed for a keyboard device, or the speed of transmissions necessary for a pointing device. Several off the shelf solutions are available to solve the problem with a range of protocols, however the majority of them are designed to interface with an additional designated processor, which is prohibitively expensive.
A current physical layer format utilizes a four pulse position modulation (4 PPM) data encoding technique prior to transmitting the infrared signal. The pulse position modulation encoding is achieved by defining a data symbol interval and subsequently subdividing the data symbol interval into equal time slices called “chips”. In PPM schemes, each chip position within a data symbol represents one of the possible bit combinations. Logical “1” represents a chip interval when the transmitting light emitting diode (LED) is emitting light, while a logical “0” represents a chip interval when the LED is off. Thus, 4 PPM data encoding includes four chips corresponding to 4 equal time segments. For example, a symbol duration of 500 nsec has four 125 nsec chip intervals.
The following Table 1 correlates each data bit pair with a corresponding 4 PPM data sequence, or symbol.
TABLE 1Data Bit Pair (DBP)4 PPM Data Symbol001000010100100010110001
Because there are four unique chip positions within each symbol in 4 PPM, four intervals exist in which only one chip is logically a “one” while all other chips are logically a “zero”. As such, each symbol represents two data bits or a “data bit pair.” In this manner, a packet containing illustratively 40 bits representing a letter, number, or command selected by a user on the keyboard, is represented by 20 symbols modulated onto a carrier and transmitted from a infrared serial port on the keyboard to the set-top box.
A problem with signal detection by a remote control receiver, particularly with regard to the 4 PPM scheme, has been observed when two infrared pulses occur back to back. For example, when data bit pairs 11 and 00 are sent sequentially such that the chip sequence is 00011000. This occurs in instances where hardware that generates interrupts is only capable of generating an interrupt on a rising edge or on a leading edge of an incoming signal, and does not know the length of the pulse. In such instances, the IR receiver encounters difficulty in distinguishing the symbols, which results in an increase in the bit error rate (BER). Thus, there is a need for an alternative format to the traditional IR formats for devices such as keyboards and pointing devices, which cannot be operated in the traditional format, such as the 4 PPM scheme. Furthermore, there is need for a transmission technique that improves the BER, and a format that operates without a dedicated IR receiver microprocessor.